Processes for impregnating a blowing agent, such as propane, butane or pentane, into polyolefin particles composed of polyolefin resin with at least a part of a vinylaromatic resin chemically bound thereto are described in U.S. Pat. Nos. 3,959,189 and 4,168,353. In these processes, the impregnation of the blowing agent is prepared by a step of providing a suspension of the polyolefin particles, a step of washing the particles with water, and a subsequent dehydrating step for removing a suspension stabilizer deposited on the surface of resulting foamable polyolefin particles after completion of the impregnation. Since these water washing and dehydrating steps are conducted under normal pressures, part of the impregnated blowing agent escapes during these steps, which results in a reduction in the expansion ratio of the resulting impregnated particles.
In colored foamable polyolefin particles prepared by addition of a dye or the like, defects have been encountered in that the efficiency of impregnating the dye is poor and in that washing of an impregnation vessel is necessary after completion of the impregnation, due to adhesion of the dye onto the walls of the vessel.